shells/mksh: Generate man-compatible page

Currently the mksh man page will not display on a base FreeBSD install,
stating that groff is required to format the page:
$ man mksh
This manpage needs groff(1) to be rendered
First install groff(1):
pkg install groff
Renames the src mksh.1 and passes it through mandoc to generate a
man-compatible page to then be installed with the package.

PR: 286190
Approved by: Rares Aioanei <schaiba@gmail.com> (maintainer)